Source-tracking unification

  • Authors:
  • Venkatesh Choppella;Christopher T. Haynes

  • Affiliations:
  • Indian Institute of Information Technology and Management-Kerala, Thiruvananthapuram, Kerala 695 581, India;Computer Science Department, Indiana University, Bloomington, IN 47405, USA

  • Venue:
  • Information and Computation
  • Year:
  • 2005

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Abstract

We propose a path-based framework for deriving and simplifying source-tracking information for first-order term unification in the empty theory. Such a framework is useful for diagnosing unification-based systems, including debugging of type errors in programs and the generation of success and failure proofs in logic programming. The objects of source-tracking are deductions in the logic of term unification. The semantics of deductions are paths over a unification graph whose labels form the suffix language of a semi-Dyck set. Based on this idea of unification paths, two algorithms for generating proofs are presented: the first uses context-free labeled shortest-path algorithms to generate optimal (shortest) proofs in time O(n^3) for a fixed signature, where n is the number of vertices of the unification graph. The second algorithm integrates easily with standard unification algorithms, entailing an overhead of only a constant factor, but generates non-optimal proofs. These non-optimal proofs may be further simplified by group rewrite rules.